Dihydroxyacid dehydratase (DHAD) is an enzyme that catalyzes the conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate and of 2,3-dihydroxy-3-methylvalerate to 2-keto-3-methylvalerate. This enzyme plays an important role in a variety of biosynthetic pathways, including pathways producing valine, isoleucine, leucine and pantothenic acid (vitamin B5). DHAD also catalyzes the conversion of 2,3-dihydroxyisovalerate to α-ketoisovalerate as part of isobutanol biosynthetic pathways disclosed in commonly owned and co-pending US Patent Publication Nos. 2009/0226991 and 2010/0143997: In addition, biosynthetic pathways for the production of 3-methyl-1-butanol and 2-methyl-1-butanol use DHAD to convert 2,3-dihydroxyisovalerate to α-ketoisovalerate and 2,3-dihydroxy-3-methylvalerate to 2-keto-3-methylvalerate, respectively (Atsumi et al., 2008, Nature 451(7174): 86-9).
DHAD is an essential enzyme in all of these biosynthetic pathways, hence, it is desirable that recombinant microorganisms engineered to produce the above-mentioned compounds exhibit optimal DHAD activity. The optimal level of DHAD activity will typically have to be at levels that are significantly higher than those found in non-engineered microorganisms in order to sustain commercially viable productivities, yields, and titers. The present application addresses this need by engineering recombinant microorganisms to improve their DHAD activity.